Remove one more manual change signal

Missed in the previous round, and the reason why the settings dialog
sometimes started in a dirty state, which depending on KCM load order
might show you the message box warning about leaving a dirty page.

This then gets much worse by TextCustomEditor::PlainTextEditorWidget
emitting change signals with a single shot timer when its highlighter
kicks in (despite the content not actually changing). Those signals
are processed in the sub event loop of the above mentioned message box,
keeping us in a loop for as many pages with those widgets there are.

The latter is pre-existing and isn't addressed by this change, it merely
hides it back under the carpet again, like it was before the
KConfigDialogManager changes.